This invention relates to a method of providing improved antiperspirant effectiveness and stability for basic aluminum chloride compositions (also known as aluminum chlorhydroxide compositions) and to such improved compositions. It has hitherto been proposed to increase the antiperspirant effectiveness of aluminum chlorhydroxide by aging under specified conditions an aqueous solution containing the aluminum chlorhydroxide, as described in Gosling et al. U.S. Pat. No. 4,359,456 and in British Patent Application No. 2048229A until the solution exhibits specified characteristics. In the British application it is pointed out that the increased activity is the result of a group of complexes called Al.sup.c' characterized by having a diffusion constant in gel-permeation chromatography which is within the range generally found for the Al.sup.b group of complexes but which displays a complexing rate in the ferron test which is in the range of the Al.sup.c complexes and by having molecules which are less than 100 A in size in aqueous solution. The Al.sup.c' complexes thus prepared are stable in aqueous solution at concentrations in the range of 10% to 30%.
It has also been proposed to increase the antiperspirant effectiveness of aluminum chlorhydroxide compositions by including in them zirconyl hydroxy chloride, and further by subjecting aqueous solutions of such compositions to heating under certain conditions, then rapidly drying the solution to solid form. The product when dissolved in water loses its improved efficacy rapidly, within a matter of days. The present invention also provides a method for improving the stability or efficacy, of compositions containing both aluminum chlorhydroxide and zirconyl hydroxy chloride, and provides such improved compositions.
It has now been found that a large increase in antiperspirant effectiveness of basic aluminum chloride (or aluminum chlorhydroxide) is provided by mixing it with monosilicic acid in aqueous solution, the amount of the chloride being 5-20% by weight and the amount of monosilicic acid being 0.03 to 8% by weight, preferably 0.3 to 3%, based upon the weight of the total mixed solution. The molecular distribution characteristics of the basic aluminum chloride immediately begin to change as determined by size exclusion (SE) chromatography, and at the same time the antiperspirant effectiveness begins to improve; the improvement continues at a gradually decreasing rate at room temperature for a period of a week or more, after which the composition retains its improved efficacy indefinitely, whether the composition is in the form of an aqueous solution or whether it is dried to a solid by removing the solvent. In dry solid form the composition contains basic aluminum chloride and silica in proportions as low as a few parts per million of the latter, based on the total.
The basic aluminum chloride may be added in dry powder form to the freshly prepared monosilicic acid aqueous solution with stirring, or if desired the basic aluminum chloride in aqueous solution may be mixed with the acid solution. The presence of zirconyl hydroxy chloride along with basic aluminum chloride either during or after treatment of the latter in accordance with the present invention is not deleterious. However, it is preferred to add the zirconyl hydroxy chloride after the basic aluminum chloride has been mixed with monosilicic acid in solution because if the zirconyl compound is introduced earlier the solutions mixture tends to gel, either immediately or within a limited time. In gel form the mixture is more difficult to process, e.g., by drying, into a form suitable for incorporating into a commercially preferred formulation. In some cases, however, for example when the monosilicic acid is prepared by hydrolysis of tetraethyl orthosilicate as described hereinafter, there is sufficient delay in the onset of gelation caused by the presence of zirconyl hydroxy chloride during the interaction between the basic aluminum chloride and the monosilicic acid so that there is adequate time to subject the solution to drying before it gels, thus providing the composition in stable dry, solid form.
Preferably the zirconyl hydroxy chloride, when introduced, is in the form of 5% to 25% by weight aqueous solution, although it can also be used in the form of a dry solid powder. The mixture of aluminum chlorhydroxide and monosilicic acid with which the zirconyl hydroxy chloride is mixed may also be either in the form of an aqueous solution or in dry solid form; the two solids may be mixed in powder form, or one or both may be in aqueous solution. Indeed, the zirconyl hydroxy chloride may be formed in situ by providing a mixture of aluminum chlorhydroxide and monosilicic acid having the desired molecular distribution characteristics and adding to it hydrochloric acid (and glycine if desired) and hereafter dissolving basic zirconium carbonate in the mixture. There is nothing critical about either the concentration of the zirconyl hydroxy chloride in solution or the Zr:Al atomic ratio, although at least about one atom of zirconium for each six atoms of aluminum is required to produce a substantial improvement in the antiperspirant effectiveness of the mixture of aluminum chlrohydroxide with monosilicic acid, and little or no further improvement is produced by increasing the amount of zirconium above one atom of zirconium for each atom of aluminum in the product. In addition, an anti-gelling agent in the form of a neutral amino acid such as glycine may be present, if desired, in either the mixture of aluminum hydroxychloride with monosilicic acid or in the solution of zirconyl hydroxy chloride. The presence of such an antigelling agent is not required, however. On the other hand, if the zirconyl hydroxy chloride is mixed with the aluminum chlorhydroxide before monosilicic acid is introduced, gelation occurs whether or not such an antigelling agent is present, making it difficult to obtain the product in the desired form.
The compositions of zirconyl hydroxy chloride and aluminum chlorhydroxide (basic aluminum chloride) which can be employed in the present invention include those in which the zirconyl compound has the formula ZrO(OH).sub.x Cl.sub.y where X+Y=2 or a hydrate thereof and the aluminum compound has the formula Al.sub.2 (OH).sub.6-n Cl.sub.n where n is from 0.8 to 2 or a hydrate thereof, n preferably being 1. The atomic proportion of zirconium to aluminum in the composition may vary from about 0:1 to 1:1, and as pointed out above is preferably from 1:1 to 1:6. The atomic proportion of metal to chloride may vary from about 0.9 to 1.9. The inclusion in the composition of a neutral amino acid as described in Grad U.S. Pat. No. 2,854,382, which patent is incorporated herein by reference, does not have an adverse effect upon the formation of the desired novel complex. Commercially available compositions of zirconyl hydroxy chloride and aluminum chlorhydroxide mixtures frequently contain glycine as the neutral amino acid in an amount such that the molar ratio of glycine to zirconyl hydroxy chloride is about 1:1. The glycine or other neutral amino acid aids in preventing gelation of the aqueous solution before or during the heating step of the method of the present invention; and its presence does not prevent formation of the desired complex and does not interfere with the conversion of the heated solution to solid form nor with the subsequent use of the solid as an antiperspirant.
The monosilicic acid in solution should be freshly prepared; by this is meant an aqueous solution containing the acid in dissolved form, substantially free from precipitate and from gel. The precise amount of monosilicic acid required to convert conventional basic aluminum chloride to the new form having greatly enhanced antiperspirant effectiveness varies depending not only upon the concentration of the chloride but upon the procedure used to prepare the monosilicic acid solution and upon the length of time elapsing between its preparation and its use. For optimum results, the solution of monosilicic acid should be used as soon as possible after its preparation. Depending upon temperature of storage as well as upon pH and concentration, the monosilicic acid solution tends to lose its effectiveness after storage for a few days or weeks, presumably because of progressive polymerization of the monosilicic acid. Water-insoluble or precipitated silica, e.g., colloidal silica, or silica gel, is ineffective for the purpose of this invention, as is an alkaline solution of a silicate such as sodium silicate.
It has been reported in the prior art that freshly prepared aqueous solutions of silicic acid exist in "true solution". See Otterstedt et al., J. Colloid and Interface, Sci., Vol. 115(1), 95-103 (1987) and Iler et al., J. Phys. Chem., Vol. 57, 604 (1953). These solutions are initially composed of monomeric silicic acid units. The species encountered in these solutions appear to be labile and changes in concentration or pH are followed by rapid changes in species distribution and polymerization. The silicic acid will eventually precipitate as silica until the soluble species are reduced to about 10.sup.-3 M (the solubility constant for amorphous silica).
The rate of silica formation and precipitation depends upon initial concentration of the monosilicic acid in solution, pH, temperature, the presence of cations other than hydrogen, and the presence of miscible solvent other than water, such as lower alkanols. Increases in any of the foregoing except the last tend to increase the instability of the solution and accelerate precipitation or gel formation. The presence of basic aluminum chloride dissolved in the solution of monosilicic acid increases the pH to about 3.5 to 4.
While applicants do not wish to be bound to a particular theory of interaction between the basic aluminum chloride and monosilicic acid, it is believed that competition between the monosilicic acid and the basic aluminum chloride for hydroxide ions in aqueous solution at a pH from 2 to 4, when the solution contains a very low concentration of free hydroxide ions, causes redistribution in the basic aluminum chloride to provide the molecular form which displays increased antiperspirant effectiveness.
The monosilicic acid solution can be prepared by various conventional procedures, such as the one described in Alexander, J. Am. Chem. Soc., Vol. 75, pp. 2887-8 (1953). In one preferred embodiment of the invention it is prepared by treating an aqueous solution of soluble silicate salt such as sodium or potassium silicate with a water-insoluble cation exchange resin in its hydrogen form as described, for example, in U.S. Pat. No. 2,588,389. In another preferred embodiment it is prepared by hydrolysis of tetraethyl orthosilicate. Mixing or stirring of tetraethyl orthosilicate with water at room temperature suffices to produce hydrolysis, but heating accelerates the hydrolysis as does the addition of an acid, e.g., a mineral acid such as hydrochloric.
The basic aluminum chloride may be added in dry powder form to the freshly prepared monosilicic acid aqueous solution with stirring, or if desired the basic aluminum chloride in aqueous solution may be mixed with the acid solution.
The time required to achieve optimum conversion of basic aluminum chloride to the desired product after mixing with an aqueous monosilicic acid solution may vary from several hours to several weeks, depending upon the nature of the preparation of the monosilicic acid, the extent and conditions of its storage before use; and the relative concentrations of monosilicic acid and of the basic aluminum chloride. The temperature may range from 5.degree. to 100.degree. C., preferably from room temperature to 80.degree. C. as shown in the examples. The zirconyl hydroxy chloride component optionally may be added after the desired conversion, after which the product may be treated in the same way as the product containing no zirconium hydroxy chloride.
Once the desired conversion has been achieved, the water and other volatile components such as alcohol, if present, may be evaporated if desired. The length of time the solution is allowed to stand before drying, as well as the speed of drying are not critical since the product continues to maintain indefinitely its superior antiperspirant effectiveness, once attained, whether it is in solution or in the form of dry particles, and at moderately elevated temperatures as well as at lower temperatures, even below 0.degree. C. If colloidal or precipitated material has formed in the mixed solution it may be removed by filtration for example, although some decrease in antiperspirant effectiveness may result, whether or not the composition is subsequently dried to solid form.
If a product containing both zirconium and aluminum is desired, an aqueous solution of zirconyl hydroxy chloride, with or without an antigelling neutral amino acid such as glycine, may be mixed with the dry solid product containing aluminum chlorhydroxide and monosilicic acid. Even after filtration residual silicic acid remains in the composition, either in the form of monosilicic acid or as its polymerization products although its amount is greatly reduced from the proportion present in the initial mixture of basic aluminum chloride and monosilicic acid and may even be as low as of the order of parts per million of the basic aluminum chloride. The addition of colloidal silica or silica gel to the composition has no effect upon the antiperspirant effectiveness of the composition.